Method For Producing Salts With A Reduced Water Of Crystallisation Content

ABSTRACT

The invention relates to a method for drying salts with a water of crystallisation content in convective apparatuses that can additionally be indirectly heated. The invention is primarily characterised in that the drying process takes place with a moisture content of the drying gas above a determined level. The gas surrounding the salt particles during the drying process therefore has a specific humidity. In this way, the rate of drying is positively influenced.

The invention relates to a method for drying salts containing water ofcrystallisation in convective devices.

Many salts form hydrate stages during crystallisation. The embeddedwater of crystallisation is often undesirable because it increases thetransport costs, reduces the concentration and in some cases results ina product that is not stable when stored. The extent of the hydratestage, i.e. the number of embedded water molecules, can be controlledfavourably by means of the crystallisation conditions in some cases, butin many cases there is no way round thermal dewatering. The more watermolecules embedded per salt molecule, the easier it is for the watermolecules last embedded to detach themselves again, i.e. both the energyrequired to overcome the bonding energy and the temperature required fordetachment increase as the number of water molecules already separatedper salt molecule rises. Products with a high water content, such asGlauber salt (magnesium sulphate*10 water), already gives off water ofcrystallisation at a slightly higher room temperature and liquefies(become a solution). This behaviour is the reason for the differentproduct qualities obtained after any thermal drying.

In technical practice, dusty and fine-grain products that aresusceptible to abrasion or stable granulates with low dust content andintermediate or transitions stages of both products are obtained,depending on the drying process and the operating and design conditionsselected. Drying is conducted typically in rotary dryers (drum dryers),flash tube dryers or fluidized bed dryers with or without built-in heatexchangers. Spray granulation in the fluidized bed is also used, but isless economical due to the increased water evaporation. In some cases,the drying methods are also combined with one another.

What all methods have in common is that the product quality differs andcannot be determined in advance. Many methods provide a dusty productthat is susceptible to abrasion and which has a low bulk density of800-1000 g/l, and others provide a firm granulate of 1200 g/l and more(e.g. spray granulation, fluidized bed dryers with built-in heatexchangers).

The aim of the invention is thus to produce salts with a reduced waterof crystallisation content that have certain defined propertiesregardless of the method used, where the properties depend to a largeextent on the bulk density.

The invention is thus characterised in that drying takes place above acertain defined moisture content, for example an absolute moisturecontent of 10%, in the drying gas. The gas that surrounds the saltparticles during drying thus has a certain moisture content. This has aneffect on the drying speed. At high speeds, the water evaporatesspontaneously first of all on the surface, and then more water ofcrystallisation is released inside the particle, evaporated and broughtto the surface through pores and capillaries as a result of the risingtemperature. The capillaries and pores are expanded as a result of thehigh mass flows, and a porous particle with low density and a weakenedstructure is created. At low drying speeds, the water of crystallisationthat has separated remains in a liquid state for a longer period, andthe saline solution partially formed can again close or reduce the sizeof some pores, cracks and capillaries. This results in a firm particleof high density.

An advantageous development of the invention is characterised in thatthe moisture content of the drying gas is set above the defined moisturevalue by the evaporated water alone, where the moisture content in thedrying gas can be set by adapting the water evaporation rate with theaid of the drying gas temperature or, as an alternative, by means ofadditional heat exchanger surfaces.

An alternative embodiment of the invention is characterised in that themoisture content of the drying gas is set by recirculating some of thegas, where the moisture content of the drying gas can also be set abovethe defined moisture value by fully recirculating the gas, and theevaporated water here is discharged from the loop by means ofcondensation and the moisture content of the drying gas is set with theaid of the condenser temperature.

In particular, salts with a high water of crystallisation content, e.g.decahydrates or Glauber salt (sodium sulphate), already release water ofcrystallisation at low temperatures (well below 100°). As a result ofthe low temperatures, there is not enough evaporation and the saltliquefies or cakes.

A favourable development of the invention is thus characterised in thatthe dust and/or dried product is combined and, if necessary, mixed withthe damp salt containing water of crystallisation before drying. Theexcess free water of crystallisation can be bound by means ofrecombination to obtain a non-critical mixture that can be driedfurther. This recirculation takes place typically in a mixer thatprecedes the dryer, but can also be performed in the dryer if it has theappropriate dryer geometry.

An advantageous development of the invention is characterised in that,instead of dust and/or dried product, oversized granules ground to dustare combined with the moist salt containing water of crystallisation(“coating with powder”).

A favourable development of the invention is characterised in that themoisture content can only be set in a partial geometrical area of thedryer, e.g. in the feed zone.

When drying iron sulphate heptahydrate (ferrous sulphate) to amonohydrate using the method according to the invention, it has provedan advantage to set the absolute moisture content in the drying gas tomore than 15% when it leaves the dryer in order to obtain a product witha bulk density of more than 1100 g/l.

According to the theory of the method proposed, there are severalpossible ways of setting the advantageous conditions. In the dryingunits used, the gas carries off the evaporated water, and the energy canalso be supplied via the gas and/or through contact surfaces.

In purely convective devices, the moisture content in the waste gas isgenerally too low, and the hot gas temperature and flow rate must,therefore, be selected with the aid of the energy balance such that thewater thus evaporated raises the moisture content in the waste gas tothe desired level. This does not necessarily succeed with all types ofsalts and dryers. There are limits when increasing the temperature(damage to product, material strength, energy sources available, . . .), and any increase in the volume flow does feed more energy to thedevice, but it does not alter the status of the waste gas in the desireddirection.

Convective devices with indirect heating can set the water evaporationrate independently of the volume flow, using the temperature of theheating surfaces on the one hand and the size of these surfaces on theother hand. For a fluidized bed dryer with built-in heat exchangers, forexample, this means increasing the transfer surface area by increasingthe packing density (m² of heat exchanging surface area/m³ layer volume)or increasing the layer depth at unchanged packing density if thecapacity to increase the temperature has already been exhausted. As aresult, water evaporation and moisture content in the waste gas areraised as required at unchanged inflow.

Furthermore, it is possible to set the waste gas to any desired statusby recirculating some or all of the waste gas. If it is partiallyrecirculated, some of the moist gas is discharged and the rest isrecirculated. The ratio of the gas flows determines the waste gasmoisture content and all of the evaporated water is discharged with thepartial flow.

In the event of full recirculation in a closed gas loop, the evaporatedwater is condensed in a condenser and discharged from the loop. Thetemperature of the condenser determines the moisture content in thewaste gas.

Embodiment Example 1

Iron sulphate heptahydrate with surface moisture is dried to amonohydrate in a fluidised bed dryer with a built-in heat exchanger inthe product layer. 420 kg/h of moist heptahydrate and 260 kg/h of driedrecirculated material (monohydrate product and dust from the dryingplant) were fed to the dryer via a mixer.

The drying air (1300 kg/h) had a temperature of 185° C., the heatexchanger 195° and the product layer 117° C., Together with the initialloading of 10 g/kg dry air, this resulted in an exhaust air load of 146g/kg dry air, corresponding to an absolute moisture content of 12.7% anda dew point of approximately 63° C., due to the evaporated water. Theresult of this drying process at low moisture content was a light, dustymonohydrate granulate with a bulk density of 710 g/l, average graindiameter of 250 microns, and dust discharge of 66% in the exhaust airduring drying.

Embodiment Example 2

In the same dryer, a mixture with the same amounts of heptahydrate andrecirculated material comprising monohydrate and dust from the dryingprocess was dried at virtually the same temperatures (air 178°, heatexchanger 195°, product layer 120°). The main difference compared toexample 1 was that the air was fully recirculated and the evaporatedwater fully condensed in a washer. Due to the changing product, theamount of air conveyed through the loop had to be increased to 1730kg/h. The washer had a temperature of 65°, which resulted in a totalload of 205 g water/kg dry air, in the air recirculated to the dryer.Together with the evaporated water, the exhaust air then contained 329g/kg after the dryer, equivalent to an absolute moisture content of24.8% and a dew point of approximately 73°. As a result of this humidatmosphere during drying, the monohydrate produced was stable and heavy,with a bulk density of 1195 g/l, an average grain diameter of 450microns and dust discharge with the exhaust air of only 15%.

In the following figures, the variants of the method are described usinga fluidized bed dryer as an example. These figures also apply by analogyto the other types of dryer mentioned above.

FIG. 1 shows a system according to the invention with a flow ofrecirculated material,

FIG. 2 shows a system according to the invention with partial gasrecirculation, and

FIG. 3 shows a system according to the invention with full gasrecirculation.

The supply air fan 10 (see FIG. 1) pushes ambient air through a heaterbattery 11 into the drying device (in this case a fluidized bed dryer)12, in which the salt can be heated optionally by the heat exchanger 13,and releases the water of crystallisation. The moist drying gas leavesthe dryer, the dust is removed from the gas in a filter or cyclone 14,then the drying gas passes through the exhaust air fan 18 into the freshair 19 after undergoing optional, additional gas cleaning. The salt 20with a moist surface and containing water of crystallisation is coatedwith the dust separated in the cyclone or filter 14 and optionally withrecirculated product in the mixer 21 and fed to the dryer 12. After thedrying process, the salt leaves the dryer 12 and the oversized grainsare removed on an optional screen 22, ground 23 and fed to the mixer 21.The accept fraction enters an overflow silo 24, from which recirculatedproduct for the mixer 21 is taken optionally by a dosing screw 25 and towhich the final product 26 is fed for further processing.

FIG. 2 shows the variant of the drying process with partial gasrecirculation in order to set a defined gas moisture content, i.e. adefined moisture content in the dryer. The product is treated in thesame way as described in FIG. 1, but is not shown here for the sake ofmaintaining clarity. The fan 10 presses the gas in the circulatingsystem through the heater battery 11, dryer 12 and dust remover 14. Apartial flow of fresh air, which is cleaned with the optional filter 15,is added in metered doses with the aid of the throttling device 16. Inthis way, the moisture content of the gas is set in the dryer 12. Thecorresponding amount of moist waste gas is discharged through thepressure-controlled throttling device 17 on the discharge side of thefan. This flow of waste gas 19 also contains the evaporated water fromthe product. The smaller the partial flow of fresh air, the higher themoisture content in the gas.

If the circulating system is completely closed (see FIG. 3), theevaporated water must be removed selectively from the circulating gas.This can usefully be achieved by condensation in a surface or a washcondenser 30. The wash water is pumped 31 through the circulating systemand the condensation heat is removed via the cooler 32. The volume ofcondensed water, which also corresponds to the volume of water that hasevaporated from the product, is removed from the process with levelcontrol 33. With the aid of the cooler 32, the temperature of the washwater can be set such that the initial moisture content needed in thecirculating gas results from the partial steam pressure of the water.The initial moisture content is calculated from the desired moisturecontent in the waste gas less the proportion generated by waterevaporation from the product. The circulating gas is conveyed in a loopby the fan 10 through the heater battery 11, the dryer 12, the dustseparator 14, and the washing condenser 30. The product route is thesame as is shown in FIG. 1, but is not repeated here for the sake ofmaintaining clarity.

The systems shown are only intended as examples and are illustratedusing a fluidized bed dryer (with or without built-in heat exchanger) asan example. However, rotary dryers (drum dryers), flash tube dryers orfluidized bed spray granulators could also be used.

1.-10. (canceled)
 11. A method for drying salts containing water ofcrystallisation, comprising: drying the salts with a hot drying gas in aconvective dryer whereby water is evaporated from the salt as the saltis dried and said evaporated water increases the moisture content of thedrying gas in the dryer; and maintaining the absolute moisture contentof the drying gas in the dryer above a predefined moisture contentvalue, using only water evaporated from said salts.
 12. The methodaccording to claim 11, wherein the moisture content in the drying gas inthe dryer is maintained above said value, in dependence on sensing thewater evaporation rate from said salts and the temperature of the dryinggas.
 13. The method according to claim 11, wherein the moisture contentin the drying gas in the dryer is maintained above said value, independence on additional heat supplied to the dryer through heatexchanger surfaces.
 14. The method according to claim 11, wherein asupply of moist salt containing water of crystallisation is fed to adevice upstream of the dryer; salt dust and dried salt product areremoved from the dryer as discharged material; and at least some of saiddischarged material is combined with said moist salt in said device andthe resulting combination is fed to said dryer.
 15. The method accordingto claim 14, wherein said discharged material includes oversize granuleslarger than a maximum size, and the method includes, grinding saidoversize granules; combining the ground granules with said moist salt insaid device; and feeding the combination to said dryer.
 16. The methodaccording to claim 11, wherein the dryer has a plurality of regions andthe moisture content value is maintained in only one of said regions.17. The method according to claim 11, wherein said absolute moisturecontent is at least 10%.
 18. The method of claim 12, wherein a supply ofmoist salt containing water of crystallisation is fed to a deviceupstream of the dryer; salt dust and dried salt product are removed fromthe dryer as discharged material; and at least some of said dischargedmaterial is combined with said moist salt in said device and theresulting combination is fed to said dryer.
 19. The method according toclaim 13, wherein a supply of moist salt containing water ofcrystallisation is fed to a device upstream of the dryer; salt dust anddried salt product are removed from the dryer as discharged material;and at least some of said discharged material is combined with saidmoist salt in said device and the resulting combination is fed to saiddryer.
 20. The method of claim 14, wherein said combining is in a mixingdevice.
 21. The method of claim 16, wherein the dryer includes a feedregion and the moisture content value is maintained in the feed region.22. The method according to claim 12, wherein said absolute moisturecontent is at least 10%.
 23. The method according to claim 13, whereinsaid absolute moisture content is at least 10%.
 24. The method accordingto claim 14, wherein said absolute moisture content is at least 10%. 25.The method according to claim 11, wherein the salt containing water ofcrystallisation is iron sulphate heptahydrate; said salt is dried to amonohydrate product having a bulk density of more than 1100 g/l; themoisture of the drying gas increases as the drying gas passes throughthe dryer until it is discharged; and the absolute moisture content ofthe drying gas is greater than 15% when it is discharged from the dryer.26. The method according to claim 25, wherein a supply of moist ironsulphate heptahydrate salt containing water of crystallisation is fed toa device upstream of the dryer; salt dust and dried salt product areremoved from the dryer as discharged material; and at least some of saiddischarged material is combined with said moist salt in said device andthe resulting combination is fed to said dryer.
 27. A method for dryingsalts containing water of crystallisation, comprising drying the saltwith a hot drying gas in a convective dryer whereby water is evaporatedfrom the salt as the salt is dried and said evaporated water increasesthe moisture content of the drying gas in the dryer; discharging themoist drying gas from the dryer into a loop that fully returns thedischarged drying gas into the dryer; in said loop, removing moisturefrom the discharged drying gas by condensation at a controllablecondenser temperature; and maintaining the moisture content of thedrying gas in the dryer above a predefined moisture content value bycontrolling the moisture content of the drying gas that returns to thedryer through said loop, in dependence on controlling the temperature ofthe condenser.
 28. The method according to claim 27, wherein themoisture content in the drying gas in the dryer is maintained above saidvalue, in dependence on sensing the water evaporation rate from saidsalts and the temperature of the drying gas.
 29. The method according toclaim 27, wherein the moisture content in the drying gas in the dryer ismaintained above said value, in dependence on additional heat suppliedto the dryer through heat exchanger surfaces.
 30. The method accordingto claim 27, wherein a supply of moist salt containing water ofcrystallisation is fed to a device upstream of the dryer; salt dust anddried salt product are removed from the dryer as discharged material;and at least some of said discharged material is combined with saidmoist salt in said device and the resulting combination is fed to saiddryer.